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The Sympathetic Nervous System (SNS) is a branch of the autonomic nervous system along with the enteric nervous system and parasympathetic nervous system. It is constantly active at a basal level to maintain homeostasis.[1] (called sympathetic tone) and becomes more active during times of stress. Its actions during the stress response comprise the fight-or-flight response.

OverviewEdit

Alongside the other two components of the autonomic nervous system, the sympathetic nervous system aids in the control of most of the body's internal organs. Stress—as in the flight-or-fight response—is thought to counteract the parasympathetic system, which generally works to promote maintenance of the body at rest. In truth, the functions of both the parasympathetic and sympathetic nervous systems are not so straightforward, but this is a useful rule of thumb.[1][2]

There are two kinds of neurons involved in the transmission of any signal through the sympathetic system; pre- and post- ganglionic. The shorter preganglionic neurons originate from the thoracolumbar region of the spinal cord (levels T1 - L2, specifically) and travel to a ganglion, often one of the paravertebral ganglia, where they synapse with a postganglionic neuron. From there, the long postganglionic neurons extend across most of the body.[3]

At the synapses within the ganglia, preganglionic neurons release acetylcholine, a neurotransmitter that activates nicotinic acetylcholine receptors on postganglionic neurons. In response to this stimulus postganglionic neurons - with two important exceptions - release norepinephrine, which activates adrenergic receptors on the peripheral target tissues. The activation of target tissue receptors causes the effects associated with the sympathetic system.[4]

The two exceptions mentioned above are postganglionic neurons innervating sweat glands—which release acetylcholine for the activation of muscarinic receptors - and the adrenal medulla. The adrenal medulla develops in tandem with the sympathetic nervous system, and acts as a modified sympathetic ganglion: synapses occur between pre- and post- ganglionic neurons within it, but the post ganglionic neurons do not leave the medulla; instead they directly release norepinephrine and epinephrine into the blood.[5]

FunctionEdit

The sympathetic nervous system is responsible for up- and down-regulating many homeostatic mechanisms in living organisms. Fibers from the SNS innervate tissues in almost every organ system, providing at least some regulatory function to things as diverse as pupil diameter, gut motility, and urinary output. It is perhaps best known for mediating the neuronal and hormonal stress response commonly known as the fight-or-flight response. This response is also known as sympatho-adrenal response of the body, as the preganglionic sympathetic fibers that end in the adrenal medulla (but also all other sympathetic fibers) secrete acetylcholine, which activates the secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from it. Therefore, this response that acts primarily on the cardiovascular system is mediated directly via impulses transmitted through the sympathetic nervous system and indirectly via catecholamines secreted from the adrenal medulla.

Science typically looks at the SNS as an automatic regulation system, that is, one that operates without the intervention of conscious thought. Some evolutionary theorists suggest that the sympathetic nervous system operated in early organisms to maintain survival as the sympathetic nervous system is responsible for priming the body for action.[6] One example of this priming is in the moments before waking, in which sympathetic outflow spontaneously increases in preparation for action.

OrganizationEdit

Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in the intermediolateral cell column (or lateral horn), beginning at the first thoracic segment of the spinal cord and are thought to extend to the second or third lumbar segments. Because its cells begin in the thoracic and lumbar regions of the spinal cord, the CNS is said to have a thoracolumbar outflow. Axons of these nerves leave the spinal cord in the ventral branches (rami) of the spinal nerves, and then separate out as white rami (so called from the shiny white sheaths of myelin around each axon) which connect to two chain ganglia extending alongside the sympathetic nervous system.[7]

In order to reach the target organs and glands, the axons must travel long distances in the body, and, to accomplish this, many axons link up with the axon of a second cell. The ends of the axons do not make direct contact, but rather link across a space, the synapse.

In the SNS and other components of the peripheral nervous system, these synapses are made at sites called ganglia. The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber leaves the ganglion is called a postganglionic cell. As mentioned previously, the preganglionic cells of the SNS are located between the first thoracic segment and third lumbar segments of the spinal cord. Postganglionic cells have their cell bodies in the ganglia and send their axons to target organs or glands.

The ganglia include not just the sympathetic trunks but also the cervical ganglia (superior, middle and inferior), which sends sympathetic nerve fibers to the head and thorax organs, and the celiac and mesenteric ganglia (which send sympathetic fibers to the gut).

Information transmissionEdit

Messages travel through the SNS in a bidirectional flow. Efferent messages can trigger changes in different parts of the body simultaneously. For example, the sympathetic nervous system can accelerate heart rate; widen bronchial passages; decrease motility (movement) of the large intestine; constrict blood vessels; increase peristalsis in the esophagus; cause pupil dilation, piloerection (goose bumps) and perspiration (sweating); and raise blood pressure. Afferent messages carry sensations such as heat, cold, or pain.

The first synapse (in the sympathetic chain) is mediated by nicotinic receptors physiologically activated by acetylcholine, and the target synapse is mediated by adrenergic receptors physiologically activated by either noradrenaline (norepinephrine) or adrenaline (epinephrine). An exception is with sweat glands which receive sympathetic innervation but have muscarinic acetylcholine receptors which are normally characteristic of PNS. Another exception is with certain deep muscle blood vessels, which have acetylcholine receptors and which dilate (rather than constrict) with an increase in sympathetic tone.

See alsoEdit

ReferencesEdit

  1. 1.0 1.1 Brodal, Per (2004). The Central Nervous System: Structure and Function, 3, 369–396, Oxford University Press US.
  2. Sherwood, Lauralee (2008). Human Physiology: From Cells to Systems, 7, 240, Cengage Learning.
  3. (2005) Gray's Anatomy for Students, 1, 76–84, Elsevier.
  4. (2007) Rang and Dale's Pharmacology, 6, 135, Elsevier.
  5. Silverthorn, Dee Unglaub (2009). Human Physiology: An Integrated Approach, 4, 379–386, Pearson/Benjamin Cummings.
  6. Robert Ornstein (1992). The evolution of consciousness: of Darwin, Freud, and cranial fire: the origins of the way we think, New York: Simon & Schuster.
  7. /synap/sympathetic.html |title=Sympathetic nervous system, from the University of Chicago}}


Nervous system

Brain - Spinal cord - Central nervous system - Peripheral nervous system - Somatic nervous system - Autonomic nervous system - Sympathetic nervous system - Parasympathetic nervous system

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ReferencesEdit


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